RaspBerry pi B rev2 - Issue while sampling a LM335 (temp. sensor) thru a MCP3208 ADC via SPI in Python 3 - python-3.x

Question

I tried to interface a RaspBerry pi with a LM335 temperature sensor this week-end. I'm using a MCP 3208 micro controller (channel 0) to interface the sensor. My goal is to collect samples data in SPI mode with python 3 scripts (classes).
I've checked the wiring and everything seems OK for me, I'but I'am a beginner, not really aware of Electronic concepts.
On the software side , I've installed quick2wire that claims to be python 3 compatible. In fact I want to lead the micro-controller with Python 3 API's (not thru shell calls)
Components
Raspberry pi REV2 model B with Rasbian-wheezy / Quick2wire installed. /dev/spix.y devices are listed.
MCP3208 ADC : 12 bits ADC / SPI. I'm using CS0 from the GPIO. The sensor is connected to channel 0 (B). see datasheet.
LM335 : temperature sensor. Outputs 10mV / °K. Min 5muA / Max 5 mA. It's connected to the MCP3208 channel #0 (A). see datasheet
220 ohms resistor (C). set up regarding LM335 outputs and desired temperature range coverage with my own calculations : May be a problem ...
Schematics extract
The LM335 (zener diode like) is connected as :
Wiring
Components are wired as shown bellow. Note that the yellow link is connected behind the cobbler kit on the CS0 SPI channel.
Quick2wire
I use the bellow script to query the CS0/Channel 0 GPIO interface. Unfortunately, I've not found usefull informations on the quick2wire-python-api API's. I've just copy/paste an example found as it was written in the same goal. I'm not sure if it really works :
#!/usr/bin/env python3
from quick2wire.spi import *
import sys, time
try:
channel = int(sys.argv[1])
except:
channel = 0
MCP3208 = SPIDevice(channel, 0)
while True:
try:
response = MCP3208.transaction(writing_bytes(0x41, 0x13), reading(1))
print ("output = %i" % ord(response[0]))
time.sleep(1)
except KeyboardInterrupt:
break
The script outputs :
output = 0
output = 0
output = 0
output = 0
output = 0
....
The result is the same with the channel 1 ( with argv = 1)
As the MCP3208 Din (probe output) receives voltage (see bellow) quick2wire should read at 18°C (rawghly my home inside temperature today)
3,3 V / 2^12 = 805 muA as I understand as "digital step"
18°C + 273°C = 291 => 2,91 V on the micro controller Din pin
and then return 2 910 / 0.805 = 3 615
Am I wrong ?
Controls
I've no oscilloscope, the only measures I can read are :
Voltage is 2.529 V at B checkpoint and 0,5 V (+/-5%) on the other MCP3208 channels
Note : the adjust pin is not used on the LM335 so results way not be accurate but voltage is here !
Seems to be a problem on the quick2wire side I think. But which ?
Code
The quick2wire.spi.SPIDevice class lakes of détails on the transfers parameter in terms of structure, content and output response format.
def transaction(self, *transfers):
"""
Perform an SPI I/O transaction.
Arguments:
*transfers -- SPI transfer requests created by one of the reading,
writing, writing_bytes, duplex or duplex_bytes
functions.
Returns: a list of byte sequences, one for each read or duplex
operation performed.
"""
transfer_count = len(transfers)
ioctl_arg = (spi_ioc_transfer*transfer_count)()
# populate array from transfers
for i, transfer in enumerate(transfers):
ioctl_arg[i] = transfers[i].to_spi_ioc_transfer()
ioctl(self.fd, SPI_IOC_MESSAGE(transfer_count), addressof(ioctl_arg))
return [transfer.to_read_bytes() for t in transfers if t.has_read_buf]
Another question :
how to set SPI configuration values like mode, clock speed, bits per word, LSB ... and so on.
Thanks in advance for your help.

Answer 1

I know you probably intend to learn how to use the ADC, an so this isn't really an answer to your question (I will use your very rich post for sure - thanks), but I'm aware of temperature sensors that already pack data in GPIO serial line, that are best suited for the raspberry.
You really have to read this awesome tutorial, if you haven't already.